foo.c:4:5: warning: conflicting types for built-in function 'strlen' [-Wbuiltin-declaration-mismatch]
 int strlen(char s[]);
     ^~~~~~
foo.c: In function 'main':
foo.c:7:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s1);
  ^~~~
In file included from /usr/include/stdio.h:862:0,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.c:8:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s2);
  ^~~~
In file included from /usr/include/stdio.h:862:0,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.c: In function 'Concat':
foo.c:17:5: warning: array subscript has type 'char' [-Wchar-subscripts]
   s3[i]=s1[i];
     ^
foo.c:17:11: warning: array subscript has type 'char' [-Wchar-subscripts]
   s3[i]=s1[i];
           ^
foo.c:15:6: warning: unused variable 'strlens3' [-Wunused-variable]
  int strlens3=strlen(s1)+strlen(s2)-2;
      ^~~~~~~~
foo.c: In function 'main':
foo.c:7:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
  gets(s1);
  ^~~~~~~~
foo.c:8:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
  gets(s2);
  ^~~~~~~~
/tmp/ccweqWuw.o: In function `main':
foo.c:(.text.startup+0x30): warning: the `gets' function is dangerous and should not be used.

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